The preferred embodiment relates to a method for determining an operating position of an open/closed-valve positioned by a pneumatic actuating drive. The so-called open/closed-valve is generally also referred to as a safety valve that, in most designs, can adopt a completely closed or completely opened position in which the working chamber of the pneumatic actuating drive is pneumatically loaded. Upon complete loading of the working chamber the pneumatic working pressure pushes a working piston of the actuating drive against a force accumulator, such as a compression spring, into a predetermined abutment position, the force accumulator attempting to push a valve member of the open/closed-valve either into the completely opened position or the completely closed position.
The valve member is rigidly coupled to the working piston via a pivoting and/or translational mechanism, in particular via an actuating shaft or actuating rod. If an unforeseen emergency situation occurs, a plainly working plant field device without actuation control or position control, comprised of the pneumatic actuating drive and the open/closed-valve, in particular for a petrochemical plant, a large chemical plant, a pharmaceutical plant, a plant for steel and cement production, a waste incineration plant, foundries, etc., namely an IP converter pneumatically coupled to the working chamber of the actuating drive, causes a pressure release or venting of the working chamber of the pneumatic actuating drive. Thus the stored potential spring energy is released within the pneumatic actuating drive, and the valve member of the open/closed-valve is moved to the safety position.
In view of a guarantee for high operational safety required by the plant operator, it is necessary to regularly check functionality of such plant field devices or safety valves. In this check it is to be established whether and how the valve member actually moves into the safety position as a result of the emergency venting of the pneumatic actuating drive, and/or whether, for example, it remains for too long or even indefinitely in its departure position as a result of adhesion from long-term wear.
It is known to check the predetermined safety position of the open/closed-valve by means of an, in particular electrical, so-called position signal transducer that will output a respective electrical position signal only upon adoption of the safety position, and thus signals the expected adoption of the safety position. It emerged that in order to guarantee a high functional safety of the open/closed-valve, it is necessary to know the displacement over time to the safety position. If, for example, the adoption of a predetermined safety position is much delayed, this could be an indication of a prevailing or at least a beginning of wear and tear damage of the plant field device, which damage might affect all or only some particular mechanical or electrical functional components of the plant field device.
For known plant field devices with an exclusive open/closed-valve functioning, mechanical sensing devices are used for capturing the position of the valve member, which devices realize a direct sensing of the position at the valve member or a component of the actuating mechanism of the open/closed-valve of the actuating drive rigidly coupled to the valve member. In view of a high examination precision of the open/closed-valve or the actuating drive it is desirable to position a position sensing device as near as possible to the valve member in order to exclude a faulty position determination due to the failure of a coupling member of the mechanical chain between the actuating drive and the valve member. A disadvantage of this proximal, direct position sensing is that many of such plant field devices with a simple open/closed-valve function are to be used in technical process plants where stringent explosion protection regulations are to be abided with. An electrical potential in the vicinity of a plant process fluid at risk of explosion must be minimized, if not totally avoided. Furthermore, when disposing the position sensing device in the vicinity of the valve member, a frequent temperature change may have a negative effect on the measurement results insofar as the position sensors are usually adjusted and calibrated at room temperature prior to their delivery to the customer. During application at the ducts carrying the process medium usually much higher or lower temperatures prevail so that the sensors are to be readjusted with some expenditure.